A dimension line is the workhorse measurement annotation: a line with an arrowhead (or a tick or oblique dash) at each end, with the measurement value placed above or within a break in the line. The endpoints touch (or just clear) the extension lines that project outward from the feature being measured.
Dimensions go outside the view where possible, parallel to the feature being measured, and grouped from smallest to largest spacing outward from the part. CAD60's dimension placement algorithm follows AS 1100 conventions and arranges lines so they don't overlap or cross.
Every dimension line on a CAD60 drawing is computed from the 3D model. Change a parameter and every dimension recomputes with it.